There are a variety of high power microwave applications that require a power level that is in excess of the highest power available from a single microwave source. To fulfill such needs, the logical approach is to combine the power of a sufficient number of the highest power available sources. In order to satisfactorily combine sources it is necessary that they be at the same frequency and phase, i.e., that they be “phase locked”. The commonest combining method of phase locked microwave sources, well known to those skilled in the art, is via an assembly of four port hybrid components such as the “magic tee” (FIG. 1A), the side wall 3 dB coupler (FIG. 1B), or the top wall 3 dB coupler (FIG. 1C). In the case of the magic tee, if the phase locked inputs are of equal magnitude, in phase, and applied to ports 1 and 2, then the combined power of 1 and 2 will exit port 3 and no power will exit port 4. For the side wall 3 dB coupler, if the phase locked inputs are of equal magnitude, and source 2 lags source 1 by 90 degrees (−j), then the combined power of 1 and 2 will exit port 3 and no power will exit port 4. For the top wall 3 dB coupler, if the phase locked inputs are of equal magnitude and source 2 leads source 1 by 90 degrees (j), then the combined power of 1 and 2 will exit port 3 and no power will exit port 4. In all cases if the phase locked sources are not equal in magnitude and/or the phase relations are not as stated above, then some of the power will exit port 3 and some will exit port 4. The exact amount is determined by applying the respective amplitudes and phases of the sources to the appropriate scattering matrix. The discussion following refers to the magic tee, but it could equally apply to the other two combiners with appropriate consideration for their scattering matrices.
Referring to the Magic Tee in FIG. 1A, if microwave power P1 at phase θ1 is applied to port 1, P2 at phase θ2 is applied to port 2, and both are at the same frequency, then the power exiting port 3 is:
                              P          ⁢                                          ⁢          3                =                                                            P                ⁢                                                                  ⁢                1                            +                              P                ⁢                                                                  ⁢                2                                      2                    +                                                    P                ⁢                                                                  ⁢                                  1                  ·                  P                                ⁢                                                                  ⁢                2                                      ⁢                          cos              ⁡                              (                                                      θ                    ⁢                                                                                  ⁢                    1                                    -                                      θ                    ⁢                                                                                  ⁢                    2                                                  )                                                                        (                  EQ          .                                          ⁢          1                )            And the power exiting port 4 is:
                              P          ⁢                                          ⁢          4                =                                                            P                ⁢                                                                  ⁢                1                            +                              P                ⁢                                                                  ⁢                2                                      2                    -                                                    P                ⁢                                                                  ⁢                                  1                  ·                  P                                ⁢                                                                  ⁢                2                                      ⁢                          cos              ⁡                              (                                                      θ                    ⁢                                                                                  ⁢                    1                                    -                                      θ                    ⁢                                                                                  ⁢                    2                                                  )                                                                        (                  EQ          .                                          ⁢          2                )            
Equations 1 and 2 show that if P1=P2 and the phase difference, θ1−θ2 is zero, then the total power, P1+P2, exits port 3 and zero power exits port 4. According to Equation 2, if P1 is not equal to P2 and/or θ1 is not equal to θ2, then less than the total power exits port 3 and the remaining power exits port 4. To efficiently employ the magic tee to combine the power from two sources it is essential that both the power levels and phases applied to ports 1 and 2 are substantially equal, which is possible if the amplitudes are equal and they are phase locked.
If the complex signal amplitudes A1 and A2, corresponding to the powers P1 and P2 applied to ports 1 and 2 are not phase locked, then the power from each behaves independently and in accordance with the mathematical expression of the following scattering matrix:
      S          ^              ^        ^              :=            1              2              ·          (                                    0                                0                                1                                1                                                0                                0                                1                                              -              1                                                            1                                1                                0                                0                                                1                                              -              1                                            0                                0                              )      
Specifically, the P1 power divides equally between port 3 and port 4, and the P2 power divides equally between port 3 and port 4. No power is cross coupled between ports 1 and 2 provided that ports 3 and 4 are match terminated.
A magic tee arrangement can provide a means for combining the power from two phase locked sources. A combination of magic tees provide the means for combining the power from any number of phase locked sources with the constraint that the total number is 2n, n being an integer. The effectiveness or efficiency of the combination is determined by the application of Equations 1 and 2. It is obvious that the best results can occur when the sources are as identical as possible in both amplitude and phase.
An example of an array of 3 magic tees for the combination of 4 sources, M1-M4, is shown in FIG. 2. Ports 4 of the magic tees are terminated in matched loads, L, to dissipate the power resulting from any possible mismatches of power and/or phase of the sources. The combined power exits from port 3 of the magic tee labeled T21 and any power due to mismatches exits to port 4 into the matched termination.
In order to successfully combine oscillator power in a magic tee array, some means must be employed to accomplish the phase locking of the input sources. Phase locked sources fall into two general categories: master oscillator power amplifiers (MOPA) and phase-locked oscillators. The MOPA types include klystrons, traveling wave tubes (TWT), various cross field amplifiers (CFA) such as amplitrons, and solid state amplifiers. The phase locking of the MOPA configuration is accomplished by the master oscillator providing the same phase drive to each of the amplifiers. The present invention does not apply to MOPA type sources, but only to phase-locked oscillators. Thus in addition to the assembly of magic tees shown in FIG. 2, there must also be a means for achieving the phase locking of the four oscillators.
The theory of phase-locked oscillators is well documented in the art. Early examples of the implementation of phase locking and power combining of magnetrons have been demonstrated. The magnetron is the most common high power microwave oscillator used in a phase locked applications and has the advantage of being the most efficient of the oscillator types. The theory and practice of phase-locked oscillators extends from the early 1900's and the basic principle of operation is based on providing a cross coupling signal between the oscillators with the proper phase at a greater than threshold amplitude.
Means of providing the cross coupled signal has heretofore been a complicated array of waveguide components in addition to the power combining array. An example of such an apparatus is shown in FIG. 3. The apparatus shown in FIG. 3 includes both the power combining magic tee and the components required to accomplish the phase locking of the two magnetrons, M1 and M2. The phase locking is accomplished by the use of circulators, 5 and 6, and directional couplers 7 and 8. A sample of the power output from the magnetrons, M1 and M2, is obtained by means of the couplers 7 and 8. The sample is then injected by means of the circulators, 5 and 6, into the magnetrons M1 and M2. The magnetrons are thus cross coupled with sample signals of the appropriate amplitude and phase to accomplish phase locking Respectively referenced, the outputs of the phase locked magnetrons, M1 and M2, are passed through the circulators, 5 and 6, and then to the input ports of the magic tee, ports 1 and 2, where they combine and exit through port 3 to the load.
Other configurations for phase locking and power combining are known in the art, each requiring considerable additional apparatus to accomplish the phase locking with respect to the power combining apparatus. What is still needed in the art is a simpler and more efficient means for phase locking multiple microwave oscillators such that the power output of several oscillators can be coherently combined to achieve a single output which has the total sum power of the multiple oscillators.